1. Field of the Invention
This invention relates to a method for fabricating a semiconductor laser device.
2. Description of the Prior Art
One of typical known materials for visible light semiconductor laser devices is (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P wherein 0.ltoreq.x.ltoreq.1. Among compound semiconductors of the elements of Group III-V of the Periodic Table, (AlGa).sub.0.5 In.sub.0.5 compounds exhibit the highest band gap energy except for nitrides of the element. Accordingly, they are usually applied as a semiconductor laser device capable of emitting light or beam a red color region. For the fabrication of a semiconductor laser device using (AlGa).sub.0.5 In.sub.0.5 P, a GaAs substrate is first provided, on which the above compound semiconductors are epitaxially grown to form a compound semiconductor structure including a pair of clad layers sandwiching an active layer therebetween. The epitaxial growth is usually effected on the plane (001) of the GaAs substrate because of the ease in the preparation of such a substrate and also in the epitaxial growth. However, if GaInP is grown on the plane (001) of the substrate, a so-called "monolayer superlattice structure or ordered structure" is automatically formed wherein In atom-planes and Ga atom-planes are alternately arranged in the directions of [111 and [111] while intervening a P atom-plane therebetween.
This "monolayer superlattice structure" strongly appears in the vicinity of the growth temperature at which crystals of good quality are obtained. The completeness of the structure depends greatly on growth conditions including a feed ratio (i.e. ratio between the atoms of Group V and the atom of Group III), the growth temperature and the like. The area, in which the "monolayer superlattice" is formed, and the completeness of the arrangement depend on these conditions.
The formation of the "monolayer superlattice structure" has been observed not only in the GaInP/(001)GaAs system, but also in (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P/(001)GaAs systems. The "monolayer superlattice structure" has relation to the magnitude of band gap energy (Eg), the acceptor level (Ea) and the donor level (Ed). It is known that if the degree of the completeness of the arrangement becomes high, the values of Eg, Ea and Ed and becomes small. On the other hand, the low degree of the completeness results in large values of Eg, Ea and Ed.
In the semiconductor laser devices using the active layer and clad layers using GaIn or AlGaInP compounds, when the monolayer superlattice structure is formed, there appears a phenomenon where the lasing wavelength becomes longer than as expected from the layer compositions. For instance, with GaInP which is lattice-matched with a GaAs substrate, the band gap energy amounts to 1.91 eV, which corresponds to a wavelength of 649 nm. The lasing wavelength of a semiconductor laser device which has been fabricated using the GaInP ranges approximately from 670 to 680 nm. This means that the band gap energy becomes smaller by from 50 to 90 meV. The difference in the wavelength corresponds to a difference in the visibility by about ten times. This presents one of problems to solve under circumstances where there is a strong demand for shorter lasing wavelength. The problem is also involved in the case where AlGaInP is used as the active layer.
If the clad layer consisting of a GaInP or AlGaInP compound is doped at high concentrations, a so-called "disordering" takes place to destroy the "monolayer superlattice structure, resulting in crystals having an ordinary band gap energy. This entails larger values of Eg, Ea and Ed with the reduction of an apparent activity of the dopant. For example, AlInP is grown on a GaAs substrate according to gas source molecular beam epitaxy (GSMBE) while doping with a P-type beryllium dopant. The relation between the electrical activity (i.e. the ratio in concentration between the acceptor and Be) and the concentration of Be is shown in FIG. 1. The figure reveals that as the concentration of Be is increased, the electrical activity is abruptly decreased. The reason for this is considered as follows: the "monolayer superlattice structure" is disordered when the concentration of Be is in the range of from 1.times.10.sup..about. to 2.times.10.sup..multidot. cm.sup.-3, so that the acceptor level becomes deep and thus, thermal excitation of positive holes takes place satisfactorily with the concentration of the holes being not increased.
For the fabrication of a semiconductor laser device, the clad layers become thick owing to the doping with a dopant. This leads to a high electric resistance in the layers to such an extent that most of the element resistance is concentrated on the clad portion. At the clad portion, the Joule heat is generated with the attendant problem that the power consumption is increased along with an increase of threshold current.
On the other hand, the control of the formation of the "monolayer superlattice structure" with the feed source ratio (element of Group V/elements of Group III) has the following problems.
FIG. 2 shows a variation curve of band gap energy. Eg, for different growth conditions used in an metalorganic vapor phase epitaxy (MOVPE). As will be seen from the figure, when the feed source ratio is made low and the growth temperature is made high (at a level higher than 700.degree. C.), the band gap energy reaches about 1.9 eV which is inherent to the compound semiconductor, GaInP. Thus, the short lasing wavelength can be realized. However, when the growth temperature is made so high that there is obtained a band gap energy of 1.9 eV, there arises the problem that the light emission efficiency is lowered due to the high temperature.